The local or "last mile" portion of telephone company networks primarily consists of copper transmission loops that were originally designed to carry relatively low bandwidth voice signals. The demand for high bandwidth data services, including internet access and video conferencing, has led to the development of digital subscriber line (xDSL) technology to serve these increased demands over the existing copper loops. The "x" in xDSL is a placeholder for referring to different versions that have been proposed in the telecommunications industry to handle different services. These different versions include asymmetric digital subscriber line (ADSL), high-speed digital subscriber line (HDSL), symmetric digital subscriber line (SDSL) and very high-speed digital subscriber line (VDSL). ADSL provides a high-speed, downstream data channel to subscribers and a lower-speed, upstream data channel to the network while simultaneously providing lifeline Plain Old Telephone Service (POTS) over a single copper loop. ADSL uses the frequency spectrum of the copper loop between 4 KHz to 2.2 MHz to carry the downstream and upstream data channels and uses the frequency spectrum between 0 KHz and 4 KHz for POTS.
FIG. 1 shows a splitter-based ADSL system which includes an ADSL modem 100 and passive splitter 102 at customer premises 103 connected over transmission loop 112 to passive splitter 114 and ADSL modem 120 at central office 105. In a splitter-based approach, the respective passive splitters 102, 114 at the customer premises and central office ends of the copper loop are used to separate low frequency POTS signals (i.e., voice) from high frequency ADSL signals. Thus, at the customer premises, ADSL modem 100 receives data signals for computer 108 through high pass filter 104 and telephone device 110 receives voice signals through low pass filter 106. At the central office end of the loop, ADSL modem 120 receives data signals for a data switch (not shown) through high pass filter 116 and voice signals are separated by low pass filter 118 to connect to a voice switch (not shown).
A difficulty with the splitter-based approach is the cost and time needed to install the passive splitter at the customer premises. An approach that eliminates the passive splitter at the customer premises is shown in FIG. 2. In the splitterless approach, ADSL modem 100A and telephone 110 can be plugged directly into any telephone jack in the home. The splitterless ADSL modem 100A includes a built-in high pass filter 104A to prevent voice frequencies from reaching the modem 100A. However, without the splitter 102 (FIG. 1), high frequency data signals generated by ADSL modems 100A, 120 can appear across the telephone 110. Although these high frequencies are above the normal range of human hearing, nonlinearities in the telephone 110 produce intermodulation products from the ADSL signals in the voice band which contribute a significant amount of noise in the telephone 110.
One approach to this noise problem is to lower the transmit power of the ADSL modems 100A, 120 which, because of the nonlinearity, causes a greater reduction in noise at the telephone 110. However, this reduction of transmit power also results in a reduction in the modem bit rate since the signal-to-noise ratio is reduced.
A telephone is either in the off-hook state (i.e., in use) or the on-hook state (i.e., idle). A further problem with the splitterless approach is that the off-hook impedance of the telephone 110 at the ADSL frequencies is quite different from on-hook conditions and varies from telephone to telephone. Although ADSL transmission protocols generally are robust enough to compensate for these impedance changes, it becomes necessary to interrupt data transmission for at least one second in order to perform modem retraining whenever a telephone switches between the on-hook and off-hook states.